1. Field of the Invention
This invention relates to the field of thermal measurement devices, and in particular to a target placement apparatus for placing targets at the objective plane in an infrared sensitivity tester or calibrator.
2. Brief Discussion of the Prior Art
Heat detecting instruments are widely known and are becoming even more popular as technology advances to produce instruments capable of detecting slight temperature differences between objects at a considerable distance from the sensor. An example of such a device is the infrared camera which is sensitive to the emission of infrared radiation from bodies, such emission varying dependent, inter alia, upon the size and temperature of a body emitting the radiation. There are a variety of types of devices for which the temperature sensing sensitivity or its minimum resolvable temperature characteristics are desired to be known. For the sake of simplicity in this description, it will be presumed that the instrument needing analysis of its sensitivity or minimum resolvable temperature is an infrared camera which focuses upon a target in an objective plane and produces an image and/or a data transmission representative of the temperature profile at the objective plane. It will be understood, however, that this invention is not limited to the measurement or calibration of infrared cameras and would apply equally well to any of a number of infrared imaging devices.
It is common practice in the art of calibrating infrared cameras, and the like, to position a target plate at the objective plane of the camera and emit heat from a heat source through apertures in the target plate, directed to the camera lens, while maintaining a highly controlled temperature differential between the target plate and the heat source. By maintaining a precise and selectable temperature difference between the heat source and the target plate and calculating the apparent angular size of the apertures in the target plate as seen by the camera, the minimum resolvable temperature difference (MRTD) as a function of resolution element size can be determined. As a result, the maximum sensitivity of the camera can be established as characteristic of that particular camera. The MRTD of an infrared camera is an important parameter for certain intended uses; it is defined as Parameter 100 by MIL-STD-1859 entitled Thermal Imaging Devices, Performance Parameters Of. For example, if the infrared camera is used to detect vehicles at a distance, the smaller the camera's MRTD, the greater the ability of the camera to resolve the existence of a vehicle at greater distances.
Of course, for the same camera, the MRTD will be smaller for larger objects and larger for smaller objects. Accordingly, a more sophisticated parameter to characterize the capabilities of an infrared camera is the modulation transfer function (MTF). MTF is defined in Parameter 102 of MIL-STD-1859, Optical Transfer Function (OTF), as the modulus of the OTF. It can be calculated from MRTD measurements taken at multiple spatial frequencies.
These are the measurements that are needed to be made in this field of art. To accomplish this, there must be a heat source that is controlled differentially with respect to a target plate normally held at ambient temperature, although the target plate could be at a controlled temperature different from ambient. The object, then, is to measure and control the temperature difference (.DELTA.T) and analyze the camera output under such controlled conditions.
Accordingly, the target plate should be uniform in temperature, and its temperature must be able to be measured critically. Normally, a platinum resistance thermometer (PRT) or other thermal sensor is placed on the source, and another thermal sensor is placed on the target plate. The temperature difference between the two sensors is then tightly controlled differentially by known thermoelectric techniques. The source is thermoelectrically controlled by a solid state heater/refrigerator such that the temperature of the source can be varied from colder than the target plate to hotter than the target plate, thereby creating either positive or negative contrast relative to the opaque or closed portions of the target plate as viewed by the infrared camera. That is, the source is either hotter than the target plate or colder than the target plate.
The source is typically an aluminum plate which is thermally controlled, and is often referred to in the art as a black body heat source.
In the interest of accuracy, it is essential to maintain the temperature of the target plate relative to the heat source in a tightly controlled manner, and further to keep the target plate from being influenced by its surroundings to the greatest extent possible. Moreover, with greater and greater sensitivities being designed into infrared cameras, the problem of measuring and maintaining the temperature of the target plate precisely and to keep the target plate free from environmental influences that can affect either the temperature of the plate or the distribution of temperature across the plate, becomes more acute. Obviously, a non-uniform temperature plate is also a source of error in measuring and/or calibrating infrared imaging devices.
One way of placing the target plate in the objective plane of an infrared camera is by manual manipulation. However, since the target plate is bound to be influenced by handling and by the proximity of the heat source, and since the slots in the target plate are often thousandths of an inch in width, the mass of the target plate is insufficient to maintain a stable temperature relative to the heat source when measuring MRT at high frequency. Accordingly, in manually placing the target plates into position, an operator would screw the edges of the target plate to a larger mass framework by means of a number of bolts and nuts around the periphery of the target plate.
There are several problems associated with such manual manipulation of the target plates, among which are easy exposure to damage by simply handling the target plates and by the tools necessary to mount the target plate to the framework. Further, the oils from the skin of the operator's hands can significantly affect the emissivity of the contaminated surface (causing cold spots) and render inaccurate readings in the infrared imaging system being analyzed. Finally, since it is often necessary to change target plates, either to select a different shape of aperture therein or to mask the heat source with slots of different dimensions representing different sized targets or targets at different distances, the manual manipulation methodology becomes burdensome, clumsy, and time consuming and results in inappropriate confidence levels in the measurements.
To overcome the difficulties associated with manual manipulation of target plates, target wheels have been used that rotate about an axle and have peripheral positions for many different target plates which can be selectively brought into the objective plane of the infrared camera in a rather quick manner.
However, even this arrangement has many drawbacks. First, as target plates become larger and larger, the size of the target wheel gets to be an impractical size. For example, for a three inch diameter target, a twenty-six inch wheel would be required to accurately index twelve targets plates into position on a selective basis. When the measurement/ calibration equipment is needed to be on site, for example in an aircraft or motor vehicle, the size of the target wheel precludes the interchange possibility of using larger targets, and therefore the ability to quickly interchange the target plates is lost. In such environments, the manual manipulation process must be used. An additional problem with the target wheel is that, as indicated previously, it is necessary to keep the target plate from changing its temperature, to the extent possible, due to the proximity of the heat source. While a target wheel is or can be rather massive to provide temperature stability for the selected target plate, there are obvious limitations as to how much mass the wheel can have and yet be operable with reasonably sized components. If a target plate in one position of the wheel is exposed to the heat source for any length of time, its temperature is raised. Since the adjacent targets are thermally coupled through the mass of the wheel, they too are influenced, as a temperature gradient is established across portions of the wheel around the selected target, again leading to measurement inaccuracies.
Yet another drawback of target wheels is that, since each selected target plate is, to some degree, thermally isolated from its surroundings, each target plate must be provided with a separate temperature sensor. This leads to inaccuracies, in that temperature sensors are very individual and unique in their thermal/electronic parameters, and thus calibration of each of the sensors becomes costly, burdensome, very time consuming, and some tolerance can be expected between targets even after calibration.
There is therefore a need in the art for a target placement apparatus which can quickly and easily position a target plate at the objective plane of an infrared imaging device, which is small in size and weight, which can quickly and repeatedly position a selected target plate in the exact same position, and which can quickly and reliably thermally couple the target plate to a window framework without manually screwing the target plate to the framework. The present invention provides a practical solution to the aforementioned problems and offers the stated benefits.